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Concord in English grammar is agreement between two parts of a sentence. For example:
1. The novel is interesting.
2. The driver has arrived.
3. My friends are here.

In other words, there is a subject-verb agreement.

There are exceptions of course. These are:
1. The boy with his girlfriend is here. (Because the word "with" replaces the word "and", the form of the verb is singular.) In this instance, one could also say, "The boy is here with his girlfriend."
2. David's friend and boss is very pleasant. (When two singular subjects refer to the same person, the form of the verb is also singular.)

Now, when a singular subject is followed by a plural modifier, it requires a singular verb. This also means that when a plural subject is followed by a singular modifier, it requires a plural verb.

Example for a singular subject followed by a plural modifier:
1. The testimony of the armed robbers was startling.

Example for a plural subject followed by a singular modifier:
2. The opening pages of the book let the reader into the world of foster children.

The world of English grammar is really an interesting one.

Goldencheese3 wrote: ↑14 Jan 2019, 22:31
2. David's friend and boss is very pleasant. (When two singular subjects refer to the same person, the form of the verb is also singular.)

I don't agree with all of your examples. For example, in the above sentence, it is not clear that you are talking about one person. The sentence would need to be rewritten slightly to convey that thought. One way to write it would be "David's friend, and boss, is very pleasant." Using an interrupter adds clarity.